import ytree
import numpy as np

tree = ytree.load('/disk12/legacy/GVD_C700_l1600n2048_SLEGAC/dm_gadget/mergertree_h5/rockstar_groups/rockstar_groups.h5')

tree2=tree[:50000]

mass_resolution=5.43*1e12

Results=np.array([np.array([mytree['prog','redshift'][mytree['prog','mass']>mytree['mass']*0.5][-1],mytree['mass']]) for mytree in tree2 if mytree['mass']>mass_resolution])